Microwave energy is utilized in microwave ovens to heat products placed within the oven cavity. One application of heating is the thermal processing of rubber. A tray used for product support in such an application should exhibit a plurality of desirable characteristics.
First, for example, the tray's mechanical characteristics must be compatible with an industrial rubber processing environment. More specifically, the tray should be resistant to the shock of being dropped or impact loaded. Also, it must be immune to temperatures up to 400.degree. F. and capable of being subjected to much higher temperatures without producing toxic by-products as, occasionally, the rubber products may burn resulting in temperatures higher than 400.degree. F. The tray should also be light and easy to handle.
Second, the tray must satisfy the constraints of microwave processing. More specifically, it must not heat in a microwave field. Also, it must be transparent to microwave energy such that it does not create significant impedance transformer characteristics to the surface of the product supported by it. It is desirable that the distribution of energy through the product be as symmetric as possible to accomplish uniformity in the heating profile. To minimize the boundary condition of the support surface, it is preferable that the material be very thin. Especially, in an oven having feeds for microwave energy at the top and bottom of the cavity, it is advantageous that the product appear to be supported in free space without impedance transformation caused by the support structure.
Third, the tray must satisfy miscellaneous requirements to be commercially successful. For example, it must be relatively inexpensive to produce and easily replaced or repaired in case of damage. Also, rubber products must not stick to the surface or combine with material; it must also be easy to clean. Finally, it is preferable that the tray provide easy access to the microwave cavity for loading and removing product.
In the prior art, products have been supported by metallic trays which may or may not be suspended from the walls of the cavity. This structure, however, has the disadvantage of not being transparent to microwave energy. Accordingly, microwave energy cannot enter the product through the supporting surface and symmetry of processing is sacrificed. Also, it is well known that metallic structures in the microwave cavity cause anomalies in the field which are difficult to predict and often reduce the uniformity of heating.
Ceramic trays were considered for the rubber processing application but they were very susceptible to breakage caused by rough handling. Furthermore, when a ceramic tray is made thick enough so as not to be so fragile, it exhibits impedance transformer characteristics at the boundary of the product which are undesirable. Also, the trays were relatively expensive.
Also considered for a tray material were a number of plastics such as polypropylene, reinforced polyesters, and polysulfone. These, however, generally had to be thicker than one-eighth of an inch to obtain the requisite strength; the required thickness impacted the impedance transformer characteristics at the boundary of the product. Primarily, these plastics are generally not suited for rubber processing environments due to temperature limitations.